Frequency discriminator



Jan. 13, 1942. K K.'RATH FREQUENCY DISCRIMINATOR Filed Feb. 14, 1941 2 Sheets-Sheet l P phasqfrequenqy Jam.v 13, 1942.y

K. RATH FREQUENCY DISCRIMINATOR Con verter' A F mplzfier Mixer A R F A mplzfier L WAIVENTOR ff/@ y Patented Jan. 13, 1942 l FREQUENCY DISCRIMINATOR l Karl Rath, New York, N. Y., assignor to Radio Patents Corporation, a corporation of 'New York Application February 14, 1941, Serial No. 378,907

(Cl. Z50-20) 18 Claims.

The present present invention relates 'to a frequency discriminator or variable yfrequency response circuit for use in automatic frequency control systems, as a converter or detector in frequency modulation receivers, for frequency monitoring and various other applications requiring means for converting frequency changes, including both the frequency of an alternating current or potential as well as the tuning frequency of a resonant impedance or network, into corresponding amplitude changes of electrical energy.

More particularly the invention is concerned with a frequency discriminator of the phase shift type comprising basically a means for deriving a pair of potentials from a varying frequency, said potentials having a phase relation varying in accordance with the frequency changes to be detected and an electron discharge tube for combining said potentials to produce Van output current or voltage vhaving an amplitude varying proportionately'to said phase variations, i. e. in turn to the frequency changes to be detected.

In frequency discriminators of the above general type as disclosed for instance in U. S.,Patents 2,268,091 to Zakarias and 2,248,197 in the name of the present applicant, difliculties are encountered in certain cases in practice due to the steady or constant plate current supplied by the converter tube upon which is superimposed a variable component being proportional to theV frequency changes being converted. Unless vthis quiescent or steady plate current of the converter is highly stabilized and maintained at a constant value, it is found that 4the operating point of the conve-rter will be shifted in accordance with fluctuations or ripples of the supply voltage or due to disturbing signals and other causes. In order to reduce the distortion .and hum as well as other defects resulting Vfrom such shifts of the operating point, adequate -smoothing filters are` required representing a drawback when price is at a premium.

The present invention has for its object to substantially overcome the above mentioned diiculties in a simple and easy mannerby providing a system requiring no steady or `quiescent `plate current for the conversion of the frequency changes into a current of corresponding amplitude variation.

Other objects and advantages of the invention will become more apparent from the following detailed description taken with reference to the accompanying drawings forming part of this specification, and wherein:

Figure 1 is a circuit diagram of one type of frequency discriminator embodying the principles of the invention,

Figure 2 is a diagram of a modied discriminator of the type according to Figure 1,

'circuit f2 I Figures `3 and 4 are theoretical diagrams ex planatory of the function and operationfo'f 'the invention,

lFigure 5 is adiagram of va frequency discriminator embodying improved features according'to the invention,

Figure 6 shows, partly in .block vdiagram form, .a discriminator according vto the invention em bodied as a converter or demodulator in a `fre quency modulation receiving system, and

Figure '7 is a circuit diagram of a frequency discriminator embodying further features of improvement according to the invention.

Like reference characters identify like lparts throughout the different views of thedrawing's.

Referring more particularly to Figure l, lthere is shown a frequency variation responsecircuit embodying the principles of the invention and comprising a vacuum Idischarge vtube I D'provided with a cathode I I, arst control grid I2, a first kpositive or screen grid I3, a second control grid I4, a second positive or screenlgrid I5., Vand a-plate I6; all arranged in the order named with respect `to the cathode. Alternating currentsignals supplied from a ysource ITI -are impressed vupon the primary circuit 2U of a double tuned 4transformer or band pass filter having a resonant secondary As is well known, such a filter represents a phase shift network producing 'a pair of voltages across the primary and secondary-windings-whose relative phase angle is dependent-upon the'relative departure of the tuning frequency -to which the circuits 20, 2| are resonant from'the frequency of the impressed alternating current supplied by the-source II. Thus, if the impressed frequency equals the resonant lfrequency of `the network, the primary and secondary ,voltages Awill `be 90 out of phase with vrespect to each other,

while with the impressed frequency departing below or -above the resonant frequencyof the `network the relative phase angle between the `primary and secondary voltages will become greater Ior less than depending upon the-frequency departure.

The potentials `developed by the primary and secondary circuits of the phase shiftnetworkare impressed each upon one of the control grids of the tube I0. To this end, in the -example shown, the primary 2li is coupled to the grid YI4 through a coupling condenser 22 and the secondary 2| is directly coupled to the grid I2 and cathode -in a manner well understood. lItem 23 represents a biasing network comprising a capacity shunted resistor in the cathode-to-ground lead of the tube to provide suitable grid biasing potential for'the grids I2 and I4 which are voperatedprefera'bly within the negative region of the operating characteristic so as to prevent a positive grid current flow and consequent energy consumption and distortioniin the grid'circuits. The screen grids I3 and I5 are operated at suitable positive potentials with respect to the cathode as indicated by the plus signs representing high potential sources, and by-passed to ground for alternating currents by means of condensers 24, 25, respectively.

The plate I6 according to the present invention is operated at cathode potential or at negative potential with respect to the cathode by the aid of the network 23. If desirable to bias the plate at a negative potential different from the negative bias of the control grids I2 and I4, an additional biasing source may be provided in the plate circuit as will be understood.

In a tube of the aforedescribed type a concentrated electron space charge or virtual cathode Will be formed in the region between the screen grid I5 and the plate i6 by virtue of the accelerating action of the screen grid I5 on the electron stream and the subsequent retarding or repelling action of the negative field produced by the plate I6. Thus, electrons having passed the screen I5 will be repelled by the plate I6 and if the latter is at a sufficiently negative potential so as to substantially prevent a steady electron current flow through the external plate circuit, the electrons will be returned and form a region of increased charge density known as a virtual cathode adjacent to and at a spacing distance from the plate. The intensity or charge density of this virtual cathode will vary in phase with the fluctuations of the electron stream emitted from the cathode, that is in phase with both signal potentials applied to the control grids I2 and I4. space charge in front of the plate I6 will vary in accordance with the vectorial sum of the control grid potentials as will be further understood from the following with reference to the theoretical diagrams shown in Figures 3 and 4.

If the impressed frequency is equal to the resonant frequency of the phase shift network 20, 2l, the potentials e1 on one control grid and 82 on the other control grid will be 90 out of phase as pointed out above. er will have a value as shown in the diagram. If the frequency of the impressed potential varies in either sense relative to the resonant frequency of the phase shift network, the relative phase Accordingly, due to this dual control the,

The vectorial resultantv of the potential e2 with respect to e1 will become Y greater or less than 90 as shown at cz and c2, respectively, producing a resultant fluctuation of the space charge cr and er", respectively, of different amplitude depending on the relative phase angle displacement between the control poten-V 1.

tials. The potential c2 not only varies in phase t;

depending on the departure of the impressed frequency from the resonant frequency of the transformer 20, 2 I', but also in amplitude in that the end of the potential vector e2 travels along a circle in a manner well known and shown in m,

Figure 3. Thus, for extreme detuning the potential ez may be neglected and the space charge will fluctuate in accordance with c1 only and detailed investigation shows that the resultant er varies according to a curve as shown in Figure 4 as a function of the relative phase angle between the potentials e1 and e2; that is, in turn of the relative departure of the impressed frequency from the resonant frequency of the network 2i), 2I. Curve er is not symmetrical with respect to the line representing the potential e1 as shown in the diagram and accordingly by suitably adjusting the resonant frequency of the network 2B, V2| in relation to the particular (center, car-J rier) frequency the relative deviations from 2G and a secondary 2'I.

which are to be detected or translated (point P), a maximum swing or range d of output current variation may be realized. The slope of the curve er representing the conversion eiciency of the frequency change into amplitude change is dependent on the changes of the phase angle as a function of the detuning between the impressed frequency relative to the resonant frequency of the network 20, 2l; that is, on the "Q Value or damping coefficient of the resonant or phase shift network. Thus, in case of low damping or high Q, the slope of the curve will be steeper (er) than in the case of high damping or a low Q (curve er) of the network or other resonant impedance means as indicated in the diagram.

From the foregoing it is seen that the tube functions in the rst place by converting frequency changes into an amplitude modulated space charge adjacent to the plate or output electrode.

Due to the fluctuations of the space charge or virtual cathode adjacent to the plate I6 at the rate of the input frequency and at amplitudes varying according to the diagram shown in Figure 4, a corresponding displacement current will be induced in the external plate circuit. This current representing an amplitude modulated signal is caused to develop a corresponding voltage by means of a resonant transformer included in the plate circuit comprising a tuned primary This voltage is rectified in any suitable manner such as by the aid of a diode rectifier structurally embodied in the tube ID. For the latter purpose the anode 28 of the diode cooperating with the common cathode I I ,f in a known manner is connected to the high potential side of the transformer secondary 2'I in series with a load resistance 30 by-passecl by a condenser 3| whereby an output voltage will be developed by said resistance varying in amplitude according to the amplitude changes of the induced displacement current in the plate circuit i. e. in turn in accordance with the initial frequency changes to be detected. The detected voltage may be further amplified or directly applied to a utilization circuit or translating device connected to terminals a-b in the drawing. The primary and secondary of the input and output transformers should be carefully shielded to prevent undesirable spurious couplings and interference liable to cause additional phase shifts and impairment of the stability and accuracy of the frequency conversion. Since the displacement current induced in the external plate circuit by the Varying space charge adjacent to the plate I6 is proportional to the rate of change of the charge density, there is provided in Figure l a by-pass condenser 29 across the diode rectifier `designed to equalize the increased induced currents for the higher frequency components, especially when the system serves as a detector for wide band frequency modulated signals. In case of a discriminator for frequency control or stabilization or for frequency monitoring, condenser 29 may be dispensed with.

From the foregoing it is seen that there is provided by the invention a frequency discriminator of the type using an electron discharge stream for producing a discriminating voltage or current, wherein no steady or quiescent output current is required. As a result, the effect of a iuctuating supply voltage and other disturbances in displaying the center or zero operating point of the system are substantially eliminated by the invention. The invention also completely eliminates the necessity of a special balancingl case the source I1 supplies a signal of varyingT frequency (uctuating carrier in case of automatic frequency control, frequency modulated radio Wave, etc.) while the phase shift network is resonant to a fixed frequency (center or carrier frequency). serve for translating changes of the resonant frequency of a circuit or network into corresponding current changes in which case the source I'I represents a constant frequency oscillator (crystal oscillator, etc.), while the tuning frequency of the network 2D, 2I is varied by either changing the effective capacity or inductance of the circuit, whereby these changes will be translated into corresponding changes of electric current in the output circuit.

Referring to Figure 2, there is shown a discriminator similar to Figure 1 but diifering from the latter in the manner of effecting the variable phase shift of the control potentials on the grids I2 and I4. The grid I2 is directly connected to the source of input signals I1 to cause an initial variation of the electron space current in accordance with the input frequency (e1). The control grid I4 is externally decoupled from the input circuit and from the remaining parts of the system and connected to ground or cathode through a resonant impedance means such as a parallel tuned circuit 32. In the same manner that a concentrated space charge or virtual cathode is formed between the accelerating gridy I5 and the decelerating plate I6, a similar virtual cathode will be formed between the accelerating grid I3 and the decelerating grid I4. This latter virtual cathode will cause a displacement current to iiow through the external circuit ofV the grid I4 containing resonant circuit 32 carefully shielded from the remaining parts of the system by means of a screen 32. The latter represents a varying impedance depending on frequency and consequently the voltage developed by it, constituting the control for the grid I4, will vary in phase relative to the space charge fluctuations or potential on the grid I2 in dependence upon the relative frequency departure of the resonant frequency of the circuit 32 from the 'input signal frequency. Since the induced displacement current varies in accordance with the rate of change of the space charge fluctuations it will be 90 out of phase with the latter and in case that the impressed frequency equals the resonant frequency of the circuit 32, the latter will offer pure ohmic impedance to the displacement current resulting in a control potential (e2) on grid I4 being 90 dephased with respect to the control potential (e1) on the grid I2. If the impressed frequency deviates in either sense from the resonant frequency of the circuit 32, the latter will offer either capacitative or inductive impedance to the displacement current and the normal 90 phase anglewill be Alternatively, the system may either increased .or decreased in substantially the samemanner as shown in Figure 3. Accordingly, therefore, the second space charge or virtual cathode formed between the accelerating grid I5 and the plate I6 will vary according to resultant er in substantially the same manner as in the case of Figure 1 and a similar operating characteristic will be obtained as shown in Figure 4. The rest of the circuit is substantially similar to that of Figure 1 as will be readily understood.

The system according to Figure 2 is especially suited for translating capacity or 4inductance changes into corresponding current changes. For this purpose it is only necessary to provide a variable inductance or capacity to form an effective tuning element of the circuit 32 and by using a circuit with a high Q value as determined by the range of inductance orr capacity changes t0 be converted, slight or vminute changes of capacity or inductance may be directly translated into substantial changes of current by means of a single tube, Where with the standard amplifiers a plurality of stages would be required to obtain a corresponding plate current swing for the same input changes.

Figure 5 shows a discriminator circuit according to the invention, wherein the tube is utilized both for converting the frequency changes into amplitude changes as well as an amplifier for the converted or output energy. For this purpose a portion of the output voltage developed by the resistor 30 is fed back upon the grid I2, whereby the cathode II, control grid I2 and screen grid I3 will function as a triode amplifier by the provision of a suitable load impedance such as a low-pass network comprising a resistance 33 bypassed to ground by a condenser 34 included in the external screen circuit. The voltage developed by the resistor 33 which in the case of a frequency modulated input signal supplied by the source II will vary in accordance with the audio or modulation signal may be directly applied to the output terminals a, b for further amplification and energizing a suitable translating device such as a loud speaker. Alternatively, the potential at modulating frequency developed on the screen I3 may be impressed by way of acoupling condenser 31 upon control grid I4 to cause additional variations of the electron stream developed in the circuit of the screen grid I5 by the provision of a further load impedance comprising resistor 35 and by-pass condenser 36. The voltage developed by the resistor 35 may serve for further utilization in a subsequent circuit or device connected to the terminals a, b. In an arrangement of this type, the controls by the grids I2 and I4 by the audio or modulation potential are in opposite phase. However, the control by the grid I4 will substantially exceed the control of grid I2 so as to secure substantial amplification while at the same time improving the stability of the system on account of the differential action of the control potentials.

Referring to Figure 6, there is shown, partly in block diagram form, a complete frequency modulation receiving system embodying a frequency converter in accordance with the invention. Frequency modulated radio signals intercepted by an antenna such as a dipole 40, 41] are passed through a radio frequency amplifier 4I and combined in a mixer 42 with local oscillations generated by an oscillator 43 to produce signal waves of intermediate frequency. The latter are further ampliiied in an intermediate frequency amplier 44 and passed through a limiter 45 of known design to remove spurious amplitude modulation. The frequency modulated potential supplied by the limiter 45 is impressed upon the grid I2 of the converter I0 of the type described in connection with Figure 2. In contrast to the latter, wherein the amplitude modulated waves in the plate circuit are detected by means of a diode rectifier embodied in the same tube, the output signal in Figure 6 is impressed upon the diode circuit of a composite screen grid-diode audio amplifier tube 46 by way of a tuned transformer comprising a primary 2B and tuned secondary 2l'. The tube 45 is provided with a cathode 4l, a control grid 48, screen grid 49 and a plate or anode U and a diode anode 5I arranged in cooperative relation to the common cathode 4l. The diode circuit includes a load resistor 52 and a grid biasing network 53 is inserted in the cathode to ground lead of the tube. Rectied potential is derived from the load resistor 52 by way of a series network comprising a condenser 54 and resistor 55 and impressed upon the control grid 4B to produce amplified rectified voltage across load resistor 55 in the plate circuit by-passed to ground by condensers 57 and 58. The amplified audio voltage is impressed upon a further audio amplifier 59 energizing a loud speaker 50.

Referring to Figure 7, there is shown a coinposite frequency-converter-amplifier according to the invention especially suited for translating slow or progressive frequency variation or for indicating tuning or frequency deviations from a predetermined value for use as a frequency, capacity, inductance deviation meter or frequency monitor and other applications. Signal waves supplied from a source of varying or constant frequency are impressed by way of resonant input circuit 2| upon the grid I2 and cathode of the tube in a manner similar to that described here-z` inbefore. If the range of frequency is limited or if only deviations from a particular frequency are to be determined as in the case of automatic frequency control or frequency monitoring systems, the Q of the resonant circuit may be chosen extremely high resulting in a corresponding narrowing of the straight line operating portion (d according to Figure 4) and considerably increased slope or conversion efliciency from frequency to amplitude change. there is shown in Figure 7 a piezo-electric crystal 6I employed as a resonant impedance means connected between control grid Iii and ground and by-passed for direct current through a portion of load resistance B5 and potentiometer 63 connected across the high voltage supply source. If desirable a separate D. C. by-pass or cathode return may be provided in the form of a high ohmic resistance 62 or the equivalent shunted across the crystal. A portion of the converted voltage developed by the resistor 3Q in the diode circuit is impressed upon the grid I2 of the tube for further amplification in a manner substantially similar to that shown in Figure 5. Screen grid I3 is connected to the positive pole of the supply source through a voltage reducing resistance 64 and to the negative pole of said source through a load resistance 65 by-passed to ground by condenser 66 to develop amplified potential varying according to the frequency changes to be detected and converted. Accordingly, a meter 68 connected in the circuit of the screen grid I3 will indicate instantly thc deviation of the input frequency from the resonant frequency of the crystal 6I and in view of the high For this purpose,

Q of the crystal 6I and the additional amplification of the direct current output voltage, extreme response sensitivities are obtained enabling the indication of deviations as low as a single cycler from frequencies of the order of one megacycle and more. If desired, the potential developed in the circuit of screen grid I3 may be impressed upon the control grid I4 by connecting the latter to a suitable tap point of the load resistance 65. By connecting the cathode or ground to an intermediate point of the potentiometer 63 there is provided a negative potential applied to the contro1 grid I4 for compensating the positive potential due to its direct or conductive connection with the screen grid I3. The resultant controls by both grids I2 and I4 which as in Figure 5 are again in opposite phase and the iinal variation may be indicated in the external circuit of the screen grid I5 by the provision of a meter 61 or may be otherwise utilized by the provision of a suitable load impedance supplying a potential for control of the tuning reactance in an automatic frequency control system or any other purpose as wil1 be understood. In place of using a pair of separately biased screen grids, a common screen grid I5 may be provided extending over both sides of the contro1 grid I4 as shown in Figure 6 and additional suppressor and other electrodes may be included in the tube in a manner well known in the art.

It will be evident from the foregoing that the invention is not limited to the specific circuits and arrangement of parts shown and disclosed herein for illustration, but that the underlying principle and novel thought are susceptible of numerous modications and variations coming within the broad spirit of the invention as defined in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A frequency discriminator comprising an electron discharge tube provided with a cathode, a pair of control electrodes and a output elec` trode, a source of high frequency waves, phase shifting means for deriving from said source a pair of control potentials having a relative phase varying in accordance with the relative departure of the frequency of said waves from a predetermined frequency, means to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge near said output electrode, means for applying each of said control potentials to one of said control elec trodes to cause said space charge to iiuctuate in accordance with the vectorial sum of said control potentials, an external circuit connected to said output electrode and cathode including load mpedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for demodulating said output energy.

2. A frequency discriminator comprising an electron discharge tube provided with means for producing an electron space current, means for producing a concentrated electron space charge in said tube, a source of high frequency waves, means including circuit connections from said source to said tube for affecting a first control of said electron space current in accordance with said high frequency waves, further control means excited from said source including phase shifting means for aiecting additional control of said space current in accordance with said high frequency Awaves at a phase varying relative to the first control phase in dependence upon the relative departure of the frequency of said waves from a predetermined frequency to cause fluctuations of said space charge in accordance with the vectorial sum of both said controls, an output electrode adjacent to said space charge biased to prevent any steady electron current flow thereto, an external circuit connected to said output electrode including load Vimpedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for demodulating said output energy.

3. A frequency discriminator comprising an electron discharge tube provided with means for producing an electron space current and a first and second control grid disposed at different points in the path of said space current, a source of high frequency waves, means for applying potential from said source to said first grid, further means including phase shifting means for exciting said second grid by potential from said source at a phase varying relative to the potential applied to said rst grid in accordance with the relative departure of the frequency of said e waves from a predetermined frequency, an output electrode within said tube, means including biasing means for said output electrode to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge in the vicinity of said output electrode. and an external circuit connected to said output electrode including load impedance means adapted to develop high frequency output energy,

and amplitude modulation detecting means for i demodulating said output energy.

4. A frequency discriminator comprising an electron discharge tube provided with a cathode, a rst and a second control grid and a plate arranged substantially in the order named, and

at least one positively biased screen grid located between said second control grid and plate, means for negatively biasing saidy plate relative to said screen grid to substantially prevent a steady electron current ow to and to produce a quency output energy, and amplitude modulation detecting means for demodulating said output energy.

5. A frequency discriminator comprising an electron discharge tube provided with a cathode, a first and a second control grid and a plate arranged substantially in the order named, and

at least one positively biased screen grid located between said second control grid and said plate, means for negatively biasing said plate relative to said screen grid to substantially prevent a steady electron current to and to -produce a concentrated electron space charge in the vicinity of said plate, a source of high frequency waves, means for impressing potential from said source upon said first control grid, resonant impedance means tuned to a predetermined frequency, the relative frequency of said waves with respect to the tuning frequency of said impedance means being variable, means for reactively exciting said second grid from said source by way of said resonant impedance means, whereby the relative phase of the exciting. potentialon said second grid will vary in proportion to the relative departure of the frequency of said waves from the tuning frequency of said impedance means, van external circuit including a load impedance adapted to develop high frequency output energy connected to said plate, and amplitude modulation detecting means for demodulating said output energy.

6. A frequency discriminator comprising an electron discharge tube provided with a cathode and a rst and a second control grid, a plate arranged in the order named, a rst positively biased screen grid located between said second control grid and rsaid plate, and a second positively biased screen grid located between said first and second control grids,-means for negatively biasing said plate relative to said first screengrid to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge in the vicinity of said plate, a source of high frequency waves, meansrincluding phase shifting means for exciting both said control grids from said source at a relative phase varying in accordance with the relative departure of the frequency of said waves from a predetermined frequency, an external circuit connected to said plate including load impedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for demodulating said output energy,

'7. A frequency discriminator comprising an electron discharge tube provided with a cathode, a rst and a second control grid and a plate, a first positively biased screen grid located between said second control grid and said plate, a second positively biased screen grid located between said rst and second control grids, means fornegatively biasing said plate relative to saidscreen grid to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge near to said plate, a source of high frequency waves, means for impressing potential from said source upon said first control grid, resonant impedance means tuned to a predetermined frequency, the relative frequency of said source with respect to the tuning frequency of said impedance means being variable, means for reactively exciting said second control grid from said source by way of said impedance means, whereby the relativephase of the control potential on said second control grid with respect to the control potential on said first control grid will vary in proportion to the relative departure of the frequency of said waves from the tuning frequency of said impedance means, an external circuit connected to said plate including load impedance means adapted to develop high frequency out-put energy, and amplitude modulation detecting means for demodulating said output en- 8. A frequency discriminator Vcomprising an electron discharge tube provided with a cathode, a pair of control grids and a plate arranged in the order named, a first positively biased screen grid located between said control grids, a second positively biased screen grid located adjacent to said plate, a source of high frequency waves, means for impressing potential from said source upon one of said control grids, resonant impedance means connected between said other control grid and said cathode, whereby said lastmentioned control grid is excited from said source by space charge coupling with the electron space current through said tube at a, phase varying in accordance with the relative frequency departure of said Waves from the resonant frequency of said impedance means, means for biasing said plate negatively with respect to said second screen grid to substantially prevent a steady electron current ow to and to produce a concentrated` electron space charge in the vicinity of said plate, an external circuit connected to said plate including load impedance means adapted to develop high frequency output energy, and amplitude modulating detecting means for demodulating said output energy.

9. A frequency discriminator comprising an electron discharge tube provided with a cathode, a first and second control grid and a plate arranged in `the order named, at least one positively biased screen grid located between said second control grid and said plate, means for negatively biasing said plate relative to said screen grid to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge in the vicinity of said plate, a source of high frequency waves, means including phase shifting means for exciting both said control grids from said source at a relative phase varying in accordance with the relative departure of the frequency `of said waves from a predetermined frequency, an external high frequency load circuit including amplitude modulation detecting means connected to said plate, further means for impressing demodulated potential from said load circuit upon one of said control grids, and load impedance means inserted in the external circuit of said screen grid adapted to develop dernodulated output energy.

10. A frequency discriminator comprising an electron discharge tube provided with a cathode, a pair of control grids and a plate arranged in the order named, a first positively biased screen grid located between said control grids, a second positively biased screen grid located adjacent to said plate, a source of high frequency Waves, means for impressing potential from said source upon one of said control grids, Vresonant impedance means connected between said other control gird and said cathode, whereby said last mentioned control grid is excited from said source by space charge coupling with the electron space current through said tube at varying phase in accordance with the relative frequency departure of said Waves from the resonant frequency of said impedance means, means for biasing said plate negatively with respect to said second screen grid to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge in the vicinity of said plate, said space charge varying according to the vectorial sum of the potentials upon said control grids an external circuit connected to said plate including load impedance means adapted to develop high frequency output energy amplitude modulation detecting means for demodulating said output energy, means to impress demodulated potential from said external circuit upon one of said control grids, and load impedance means inserted in the external circuit of said first screen grid adapted to develop demodulated output energy.

11. A frequency discriminator comprising an electron discharge tube provided with a cathode, a pair of control grids and a plate arranged in the order named, a rst positively biased screen grid located between said control grids, a second positively biased screen grid located adjacent said plate, a source of high frequency waves, means for impressing potential from said source upon one of said control grids, resonant impedance means connected between the other control grid and said cathode, whereby said last mentioned control grid is excited from said source by space charge coupling with the electron space current through said tube at varying phase in accordance with the relative frequency departure of said waves from the resonant frequency of said impedance means, means for biasing said plate negatively with respect to said second screen grid to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge in the vicinity of said plate, said space charge varying according to the vectorial sum of the potentials upon said control grids, an external circuit connected to said plate including load impedance means adapted to develop high frequency output energy amplitude modulation detecting means for demodulating said output energy, means to impress demodulated potential from said external circuit upon said first grid, load impedance means inserted in the external circuit of said rst screen grid to develop amplified demodulated potential, a coupling connection from said last load impedance to said second control grid, and further load impedance means in the external circuit of said first screen grid adapted to develop demodulated output energy.

12. A frequencydiscriminator comprising an electron discharge tube provided with means for producing a concentrated electron space charge, means for subjecting said space charge to a dual control in accordance with a common high frequency input Wave at a varying relative phase in accordance with the relative frequency departure of said wave from a predetermined frequency, an output electrode in the vicinity of said space charge biased so as to substantially prevent a steady electron current thereto, an external circuit connected to said output electrode including load impedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for deinodulating said output energy.

13. A frequency discriminator comprising an electron discharge tube provided with a cathode, a pair of control electrodes and an output electrode, a source of high frequency Waves, phase shifting means for deriving from said source a pair of control potentials having a relative phase varying in accordance with the relative departure of the frequency of said Waves from a predetermined frequency, means to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge near said output electrode, means for applying each of said control potentials to one of `said control electrodes tocause said space charge to uctuate in accordance with the' vectorial sum of said control potentials, an external circuit connected to said output electrode including load impedance means adapted to develop high frequency output energy, amplitude modulation detecting means for demodulating said output energy, and equalizing means to compensate for the increase of the current induced in said external circuit by said space charge as a function of frequency.

14. A frequency discrminator comprising an electron discharge tube provided with a cathode, a pair of control electrodes and an output elec trode, a source of high frequency Waves, phase shifting means for deriving from said source a pair of control potentials having a relative phase varying in accordance with the relative departure of the frequency of said waves from a predetermined frequency, means to substantially prevent a steady electron current flow to and to produce a concentrated electron space charge near said output electrode, means for applying each of said control potentials to one of said control electrodes to cause said space charge to iiuctuate in accordance with the vectorial sum of said control potentials, an external circuit connected to said output electrode including load impedance means adapted to develop high frequency output energy, and a rectifying circuit coupled to said load impedance means including a by-pass condenser to compensate for the increase of the current induced in said external circuit by said space charge as a function of frequency.

15. A frequency discriminator comprising an electron discharge tube provided with a cathode, a rst control grid, a first screen grid, a second control grid, a second screen grid and a plate, all arranged substantially in the order named, means for maintaining said control grids and said plate at relatively low and for maintaining said screen grids at relatively high steady `po tentials with regard to said cathode to produce a pair of concentrated space charge regions adjacent to said second control grid and said plate, respectively, and to substantially prevent a steady electron current to said plate, a source of high frequency signal energy, resonant impedance means, the relative frequency of said energy with respect to the resonant frequency of said impedance means being variable, means for impressing potential from said source upon said rst control grid, said resonant impedance means being connected between said second control grid and said cathode to develop control potential at signal frequency upon said second control grid having a phase varying in relation to the phase of the potential upon said first control grid in dependance upon the relative frequency departure of said signal energy from the resonant frequency of said impedance means, an external circuit connected to said plate and cathode including load impedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for demodulating said output energy.

16. A frequency discriminator comprising an electron discharge tube provided with a cathode, a first control grid, a first screen grid, a second control grid, a second screen grid and a plate, all arranged substantially in the order named, means for maintaining said control grids and said plate at relatively low and for maintaining said screen grids at relatively high steady potentials with respect to said cathode to produce a pair of concentrated space charge regions adjacent to said second control grid and said plate,

, respectively, and to substantially prevent a steady pendence upon the frequency changes of said signal energy, an external circuit connected to said plate and cathode including load impedance means adapted to develop high frequency output energy and amplitude modulation detecting means for demodulating said output energy.

17. A frequency discriminator comprising an electron discharge tube provided with a cathode, a rst control grid, a rst screen grid, a second control grid, a second screen grid and a plate, all arranged substantially in the order named, means for maintaining said control grids and said plate at relatively low and for maintaining said screen grids at relatively high steady potentials with respect to said cathode to produce a pair of concentrated space charge regions adjacent to said second control grid and to said plate, respectively, and to substantially prevent a steady electron current to said plate, a source of high frequency signal energy subject to frequency deviations from a predetermined frequency, resonant impedance means tuned to said predetermined frequency, means for impressing potential from said source upon said first control grid, said resonant impedance means being connected between said second control grid and said cathode to develop control potential at signal frequency upon said second control grid having a phase varying in either sense from a normal phase position in dependance upon the sense and magnitude of deviation of the signal frequency from said predetermined frequency, an external circuit connected to said plate and cathode including load impedance means adapted to develop high frequency output energy, and amplitude modulation detecting means for demodulating said output energy.

18. A frequency discriminator comprising an electron discharge tube provided with a cathode, a first control grid, a rst screen grid, a second control grid, a second screen grid and a plate, all arranged substantially in the order named, means for maintaining said control grids and said plate at relatively low and for maintaining said screen grids at relatively high steady potentials with respect to said cathode to produce a pair of concentrated space charge regions adiacent to said second control grid and said plate, respectively, and to substantially prevent a steady electron current to said plate, a source of high frequency signal energy subject to frequency deviations from a predetermined frequency, resonant impedance means tuned to said predetermined frequency, means for impressing potential from said source upon said first control grid, said resonant impedance means being connected between said second control grid and said cathode to develop control potential upon said second control grid having a phase varying in sense and magnitude relative to a normal phase position in dependence upon the direction and magnitude of deviation of the signal frequency from said predetermined frequency, an external circuit connected to said plate and cathode including load impedance means adapted to develop high frequency output energy, amplitude modulation detecting means for demodulating said output enenergy, further means for impressing potential derived from the demodulated energy upon said rst control grid, and further load impedance means connected in the circuit to said first screen grid to develop amplified demodulated energy.

KARL RATH. 

